The present invention relates generally to an improved coating method. More particularly, the invention relates to a method for coating, uniformly and at high speed, a belt-shaped support member or "a web" which is run continuously with a coating liquid such as for the manufacture of magnetic tape.
In the manufacture of magnetic tapes, a gravure coating method, a reverse roll coating method, a doctor knife coating method and an extrusion coating method have been extensively employed for coating webs with magnetic coating liquids. The coating liquid used in manufacturing magnetic tape has a relatively high density as it is a mixture of ferromagnetic material and a polymer binder and, if necessary, an additive. Accordingly, the coating liquid is thixotropic and has a high viscosity having a coefficient of viscosity of at least 2 poises measured with a commercially available Brookfield viscometer. Such a coating liquid has a special characteristic that, if it is not continuously sheared to a certain degree, its viscosity will increase and the liquid tends to form condensates. A more important consideration is that the coating liquid, which is troublesome to handle, must be applied to a web which is generally extremely thin, typically several microns to several tens of microns.
Accordingly, to coat a thin web with thixotropic coating liquid according to any one of the above-described conventional coating methods, the coating operation requires a complex technique. In addition, if the web transport speed is increased, for instance, to a speed higher than about 120 m/min in order to improve productivity, then yet further difficulties are encountered. Thus, it is rather difficult to coat a thin web with a coating liquid using the conventional coating methods.
Japanese Laid-Open Patent Application No. 84242/1977 disclosed a reverse roll system coating method. In the method therein disclosed and as shown in FIG. 1, a coating liquid 3 is applied through the nozzle 2 of an extrusion-type coating unit 1 to the surface of a coating roll 5 which is rotated in a direction opposite to the direction of movement of a web 4 which is transported continuously. Thereafter the thickness of the coating liquid 3 on the coating roll 5 is adjusted to a preferred value by a metering roll 6 which is rotated in a direction opposite to the direction of rotation of the coating roll 5. Then, the coating liquid is transferred onto the web 4 the rear surface of which is supported by a backing roll 7. The thickness of the coating liquid on the web is again adjusted and the coated film is smoothed as the web passes through the gap between a doctor backing roll 8 and a doctor blade 9.
In coating a web with the coating liquid according to the reverse roll system coating method described above the following problems arise:
(i) The web 4 is transported while being sandwiched between the coating roll 5 and the backing roll 7 which turn in opposite directions. If the speed of the web or one or both of the rolls is increased, the tension on the web upstream will be different from that on the web downstream as a result of which the web is creased longitudinally making the thickness of the coated web extremely non-uniform.
(ii) By the action of a centrifugal force, the coating liquid on the coating roll 5 has a tendency to splash in drops and to stick to the surface of the web.
(iii) As a plural rolls must be provided in the coating section, the device is intricate in construction and accordingly maintenance thereof is difficult.
Another extrusion-type coating method was disclosed in Japanese Laid-Open Patent Application No. 11575/1978. According to that method, coating liquid for a magnetic recording material is coated over the surface of a web which is floatingly supported by a fluid with the speed at which the liquid strikes the web being above a predetermined value.
In this second conventional method, the construction of the coating section was considerably simplified and therefore the difficulties described in paragraphs (i) through (iii) can be eliminated. However, the latter method is still disadvantageous in that it is difficult to floatingly support the thin web and the transport of the web is made unstable when the speed is increased with the result that it is considerably difficult to uniformly coat the web at a high speed.
Accordingly, an object of the invention is to provide a coating method in which all of the above-described difficulties accompanying a conventional coating method have been eliminated and with which a web can rapidly be coated with coating liquid with the quality of the coated film maintained high.